1. Field of the Invention
The invention relates to solar energy collectors and methods of collecting solar energy. In particular, the invention relates to a solar energy collector having both liquid and air coolant paths and methods of using such collectors in heating the enclosed space of a structure.
2. Description of the Prior Art
It is well known that solar collectors may be utilized to collect and store energy for heating enclosed spaces, supplying hot water and the like. The efficiency of a solar collector is expressed by the equation .eta.=Q/I where .eta. is the efficiency, Q is the total energy recovered from the collector and I is the insolation, i.e., the total solar energy supplied to the collector.
Solar energy collectors fall into two categories, concentrating type collectors and flat-plate collectors. The concentrating collectors typically include a curved, e.g. parabolic, reflective surface adapted for directing the parallel solar rays to a focal line. A pipe having excellent heat transfer characteristics is placed along the focal line to absorb the concentrated solar energy along that line. The concentrating collector has the obvious disadvantage of requiring a mechanical drive and control to track the sun. Furthermore, the concentrating collector does not operate well under cloudy conditions and is relatively expensive to manufacture.
Known flat-plate solar collectors utilize either liquid or air as the fluid which is driven past the collecting elements. Flat-plate collectors of the type used with closed liquid systems to heat stored liquid in a tank have the disadvantage of being of little use when the collector temperature is less than about 10.degree. to 20.degree. F. from the temperature of the liquid stored in the storage tank. Furthermore, the efficiency of a liquid type collector is strongly dependent upon the temperature of the liquid in the storage tank. Air type flat plate solar collectors overcome some of the disadvantages of the liquid type collectors; however, the low heat capacity of the air coolant necessitates the use of cumbersome and expensive energy storage means, e.g. a large volume of rocks located below the building. Furthermore, the air type collectors are inefficient in comparison to liquid-type collectors.
A review of the art shows that there is an acute need for a solar energy collector that may be used when the temperature of the stored liquid is only slightly higher than the collector temperature where, according to prior art practices, it has been uneconomical to drive the liquid through the collector to attain the relatively small temperature increase in the liquid.